The present invention relates generally to paper machines and, more particularly, to methods and apparatus for preventing the formation of an unduly high positive pressure and for maintaining the pressure at a desired level in a wedge-shaped space formed between the wall of the headbox lip beam or the like and the breast roll.
In paper machines it is conventional to support the lower lip wall of the headbox by a lower lip beam which generally comprises a box beam having a substantially triangular cross section, the "hypotenuse" of the triangular cross section being constituted by a curved wall which defines a narrowing wedge-shaped space with the breast roll of the paper machine. The forming wire, e.g., a fourdrinier wire, is passed over and moves on the breast roll.
In twin wire formers which include two breast rolls, two such wedge-shaped spaces are formed between the respective curved walls of both of the lip beams and the respective breast rolls.
The invention has applicability both to single wire as well as twin wire formers and to formers in which the direction of feed of the pulp suspension is substantially vertical as well as to formers wherein the pulp suspension is fed in a substantially horizontal direction.
The rotating breast roll or rolls and the forming wire or wires passing over the same induce a positive pressure in the wedge-shaped space or spaces during operation of the paper machine. Such positive pressure results in an air flow between the pulp suspension jet and the wire as well as through the layer of the pulp suspension. Such an air flow often results in the creation of "pin-holes" in the web being formed which adversely affects the quality of the paper produced by the paper machine.
In an attempt to overcome this problem, it has been suggested to provide large openings at both lateral sides of the wedge-shaped space through which a large quantity of air and water spray carried along by the wire can be suctioned. However, it has not been possible to alleviate the problem in this manner in actual practice.
In another attempt to overcome the problem, it has been suggested to feed water into the wedge-shaped space to thereby prevent air from entering into the space. This attempted solution has not proven satisfactory in practice since it has not been possible to completely eliminate access by air into the wedge-shaped space. Even if it were possible to prevent access of air into the wedge-shaped space by feeding water into it, such a large quantity of water would be necessary that the water discharged into the wedge-shaped space between the wire and the pulp suspension would cause the suspension to become diluted. Moreover, the injection of water into the space causes a detrimental transverse flow of the pulp suspension and resulting uneven web formation.
Indeed, it is known to those skilled in the art that the precise conditions that exist within the zone at which the pulp suspension jet is discharged onto the fourdrinier wire or between two wires are rather critical with respect to the qualitative properties of the paper being produced.